If the tattoo is a mark of a demonic creation or of a religious devotion, as the 666 would certainly be, it is forbidden, just as in the book of Leviticus. But we don't know what kind of mark the "666" is. Revelation 13.16 doesn't specify what the mark is. Some scholars think it might be a branding iron brand, some think a tattoo, and others think it's a metaphor for pledging allegiance to the beast and it won't be a literal mark on the hand or forehead. It's not specified, so I can't guarantee you that it's a tattoo.